Various commercial products, for example, electric appliances, vehicles such as automobiles, housing and building products, electric appliances, and clothes, need coating or coloring. Such commercial products are coated and colored in the manufacturing process. The appearance color is an important factor for selection of a commercial product in the sales process.
For example, with regard to automobiles, the proposed techniques of Non-Patent Literatures 1 to 5 are not yet sufficient as the countermeasure for flip flop or the like. Release of every new model car continues to increase the number of outer panel colors. The user needs for the color become diversified. New colors are often set in the case of facelift or for specially equipped cars. Automobile manufactures in Japan present several ten new colors a year. The variation of color is expanded unlimitedly and causes a difficulty in color inspection.
An RGB color system camera is conventional means of obtaining coloring information. The RGB color system has been proposed by International Commission on Illumination (CIE) to provide color matching with regard to a desired color by additive mixing of three primary colors of specific wavelengths obtained from an actual optical spectrum. An RGB color matching function is employed to indicate a spectral sensitivity corresponding to the human eye in the RGB color system. This RGB color matching function may take negative values. Additive color mixing, however, does not allow for subtraction of lights. It is accordingly difficult to handle the negative value of spectral sensitivity. The RGB color system camera deforms and modifies the negative value of the RGB color matching function for approximation. Such approximation, however, does not enable the color in the color gamut of the human eye to be accurately detected and may cause color drift or color collapse of the image or the video image. A CIE XYZ color matching function (hereinafter referred to as XYZ color matching function) has been proposed, on the other hand, as a color space by coordinate transform from the RGB color matching function, in order to eliminate the negative values of the RGB color matching function. A spectral color measurement method and a tristimulus value direct reading method are provided as means for obtaining color information using the XYZ color matching function.
The spectral color measurement method directly measures an emission spectrum emitted from a light source with multiple sensors or measures the reflectance at each wavelength in a reflectance spectrum of a sample and uses the XYZ color matching function to calculate the sensitivity, so as to obtain tristimulus values X, Y and Z with high measurement accuracy. The tristimulus value direct reading method, on the other hand, uses an optical sensor with three different filters (color sensor or photoelectric colorimeter) to directly read tristimulus values X, Y and Z as colorimetric values.
Despite these means for obtaining color information, there is still a need for means of obtaining and analyzing color information. Patent Literature 1 provides the prior art relating to this means. More specifically Patent Literature 1 aims to provide a color unevenness inspection method that easily performs color unevenness inspection and an inspection image data generation apparatus used in the color unevenness inspection method. The color unevenness inspection method includes an inspection image display process of displaying a color unevenness inspection image by a projector 2, a color space conversion characteristic obtaining process of obtaining an RGB/XYZ conversion characteristic of the projector 2, an imaging process of imaging the color unevenness inspection image with an imaging unit to obtain image data, a color space conversion process of generating converted image data by converting a second color space system of the image data into a first color space system of the projector based on the RGB/XYZ conversion characteristic of the projector 2, a converted image display process of displaying the converted image data by the projector 2, and a color unevenness inspection process of inspecting the converted image for color unevenness. Patent Literature 2 discloses determining the quality of a sample based on the ratio of overlap between original image data and masked inspection image data.
Measurement of the coating appearance of automobile shown in Non-Patent Literature 1 has been proposed for evaluation of coloring. The coating appearance includes textures such as color, glossiness, smoothness (orange peel-like skin), metallic texture and rich deep sense and defects such as roughness, convexity, concavity, dripping and shedding. Defect inspection and texture measurement have been proposed. The texture measurement is performed for the scratch, color, metallic texture, sense of build and rich deep sense. The change in color trend of automobiles and the growing interest in perceived quality are behind such measurement.
The recent trend of finish coating for automobiles is increasing the dark colors such as red and blue, instead of the light colors such as white. It is empirically known that dark colors make dirt and damage on the coating surface easily noticeable. There is accordingly a need to measure the scratch and develop a scratch-resistant paint.
The solid coating and the metallic coating have different optical properties, so that different methods of colorimetry and different measuring machines should be used for colorimetry of the solid coating and the metallic coating.
In the solid coating, the in-layer diffusely reflected light distribution of color information is isotropic. A general colorimeter is thus usable for measurement of the solid coating. The colorimeter includes a spectral colorimeter and a colorimetric color difference meter. The spectral colorimeter measures a spectral reflectance ρ(λ) of an object and computes tristimulus values X, Y and Z from a spectral distribution P(λ) of illumination light and spectral tristimulus values x(λ), y(λ) and z(λ) according to Expression (2).
The colorimetric color difference meter, on the other hand, directly measures tristimulus values X, Y and Z. Its principle is similar to the process that the human looks at an object (sample) and perceives color. An integrating sphere corresponding to the eyeball serves to collect the diffusely reflected light from the sample and lead the collected light to an optical receiver. Three types of photoreceptor cells that are placed in the retina and have sensitivity characteristics of spectral tristimulus values x(λ), y(λ) and z(λ) are replaced by the optical receiver as the combination of a color filter and a photo diode. The quantities of stimulus transmitted through the optic nerve to the cerebrum are proportional to the output of the optical receiver and are shown as X, Y and Z values by an amplifier.
The colorimetric color difference meter has the colorimetric principle similar to the human eye as described above and is small-size and compact to be suitable for the use in the field. It should, however, be noted that some coating causes metamerism by the light source. Two types of coatings appear in the same color under the natural sunlight but appear in different colors under illumination of incandescent lamp. Such phenomenon that provides color matching under a specific light source but does not provide color matching under a different light source is called metamerism by the light source. The metamerism is attributable to the difference in type of pigment or more specifically the difference in spectral reflectance between two types of coatings. Accordingly, the spectral colorimeter should be used for measurement of the color of such coating.
The metallic coating includes a glittering material in its base layer, so that its in-layer diffusely reflected light distribution shows anisotropy. This accordingly provides the phenomenon that the color looks different at different angles of incidence and different light-receiving angles, i.e., geometric metamerism. A goniospectrophotometer that is configured to change the angle of incidence and the light-receiving angle is used for colorimetry of the metallic coat. A recently proposed technique measures a spectral reflectance ρ(λ) under angular conditions and determines tristimulus values X, Y and Z.
In measurement of the metallic texture, the metallic texture denotes the sense by glittering of aluminum flakes in the coat and is also called shiny texture, sparkling texture or brightness. A proposed method scans the coat with a microscopic glossmeter and analyzes a reflective light intensity curve. But this method has not been widely used. A laser metallic texture measuring device (ALCOPE) has been developed recently and is becoming widely used.